mmc: tmio: change bus_shift to unsigned int

Sane values for bus_shift are:
   0 - for 16 bit bus
   1 - for 32 bit bus
   2 - for 64 bit bus

"unsigned long" is too much.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>